Various micro-positioning devices for generating motion are known, such as linear incremental motors or other incremental motors. As is known, a micro-positioning device includes at least one actuator comprising a “smart” material, i.e., piezoelectric materials, magnetostrictive materials, and other materials known in the art. For example, in a typical linear incremental motor (or “inchworm” device), one or more actuators typically is arranged in such a way and activated and de-activated in such a sequence as to cause the motor to take a large number of relatively small steps very quickly. Where piezoelectric actuators are used, for example, the activation is effected by way of applying appropriate voltages in an appropriate sequence.
A typical inchworm mechanism 20 is shown in FIG. 1A, and a typical complete cycle of activations and de-activations for the inchworm mechanism 20 is schematically shown in FIG. 1B. (As will be described, the remainder of the drawings illustrate the present invention.) The motion of the inchworm mechanism 20 along a guideway 21 is created by a sequence of signals fed into piezoelectric actuators 22, 24, 26 which are attached to a frame 28, and which are used for clamping and longitudinal extension. As shown in FIG. 1A, a first actuator 22 and a second actuator are attached at opposite ends 29, 30 of the frame 28, and a middle actuator 24 is positioned lengthwise between the first and second actuators 22, 26.
As is known in the art, a full cycle is composed of six steps, shown in FIG. 1B. In FIG. 1B only the actuators are shown, and shaded actuators are activated. Other actuators shown in FIG. 1B are not activated. For example, the six steps shown in FIG. 1B would result in movement in the direction of the arrow X. Such movement is generated as follows.
In the first step, the first actuator 22 is activated, causing a clamp connected thereto to clamp to the guideway 21. The middle actuator 24 is elongate, and the further elongation of the middle actuator 24 (i.e., upon activation thereof) while the first actuator 24 is activated results in the second end 30 being extended further away from the first end 29 in step 2 than in step 1 (see step 2 in FIG. 1B). In the third step, the second actuator 26 is activated, causing another clamp to clamp to the guideway 21. In the fourth step, the first actuator 22 is de-activated, causing the clamp associated therewith to release. In step 5, the middle actuator 24 is de-activated, allowing movement (i.e., displacement) of the first end 29 towards the second end 30. The direction of motion (shown by arrow X in FIG. 1B) can be changed by reversing the sequence of activations.
However, known micro-positioning devices are unable to operate at speeds which are necessary or desirable for various applications. As is known in the art, the speed of a micro-positioning device can be increased by increasing step size or increasing operating frequency of the device, or both. Higher structural stiffness of the device is desirable, as higher stiffness leads to operation at higher natural frequencies. Therefore, although increasing step size—i.e., amplifying the stroke—is desirable, it is at the same time desirable to maintain stiffness of the device.
Increasing the step size by means of including mechanical levers in a linear incremental motor has been found to be unsatisfactory because adding mechanical levers decreases the stiffness of the motor. For example, see P. E. Tenzer and R. Ben Mrad, “Amplification in Inchworm Precision Positioners”, Proceedings, 2nd CanSmart Workshop on Smart Materials and Structures, Montreal, Canada, 2001, pp. 77–84.
Although amplifying the stroke (or displacement) in a micro-positioning device is desirable in certain circumstances, de-amplifying the stroke can also be desirable. For example, at present, the closed-loop accuracy of known micro-positioning devices is limited by the accuracy of sensors in the known micro-positioning devices. De-amplification could be useful to improve closed-loop accuracy of micro-positioning devices.
It is therefore desirable to provide a micro-positioning device adapted for displacement in a direction of travel which includes a means for modifying the displacement.